Thunderbird extension : accessing filesystem and security policies

I am developing a thunderbird / firefox extension using LiveConnect and I have a two different problems in general ...

From the javaScript overlay file I use classloader to launch a .jar file ... The launched application furthermore uses another class loader to intiate another classes - all those classes are in the corresponding .jar file, but I get "unknow" exception, which can be traced back through java.net.URL.ClassLoader, java.security.AcessController and java.lang.ClassLoader ...